Neutrophil dynamics in the tumor microenvironment

Abstract

The tumor microenvironment profoundly influences the behavior of recruited leukocytes and tissue-resident immune cells. These immune cells, which inherently have environmentally driven plasticity necessary for their roles in tissue homeostasis, dynamically interact with tumor cells and the tumor stroma and play critical roles in determining the course of disease. Among these immune cells, neutrophils were once considered much more static within the tumor microenvironment; however, some of these earlier assumptions were the product of the notorious difficulty in manipulating neutrophils in vitro. Technological advances that allow us to study neutrophils in context are now revealing the true roles of neutrophils in the tumor microenvironment. Here we discuss recent data generated by some of these tools and how these data might be synthesized into more elegant ways of targeting these powerful and abundant effector immune cells in the clinic.

Figure 1. Neutrophil functions during cancer progression.

Neutrophils participate in tumor progression by acting both at primary tumors and at the (pre)metastatic niche. (A) In primary tumors, neutrophils can mediate angiogenesis through the release of MMP9, S100A8/9, and BV8 to activate VEGF. The production of growth factors and laminin degradation by the neutrophil-derived proteases neutrophil elastase (NE) and MMP9 can assist tumor cell proliferation. Alternatively, inflammatory stimuli (IL-1β and TNF-α) can induce neutrophil MET expression and binding of HGF, leading to NO production and tumor cell killing. Neutrophils also use antibody-dependent cellular cytotoxicity (ADCC) to kill cancer cells. (B) Neutrophils can support metastasis through a number of different factors individually or in combination. Inflammation induced by molecules such as S100A8 increases vascular permeability and therefore extravasation. Direct interactions between cancer cells and neutrophils or NETs can lead to their arrest in the vasculature. In addition, NETs have been suggested to wake dormant tumor cells, and neutrophils can feed tumor cells with lipids to aid their survival. Together, these events favor tumor cell extravasation and metastasis. Neutrophils can also aid tumor cell killing. CCL2 produced by the primary tumor can activate neutrophils in the premetastatic niche to produce hydrogen peroxide, providing an efficient tumor cell killing mechanism. IFN-β has also been shown to increase neutrophil antitumor potential by increasing NET capacity and cytotoxicity toward tumor cells. (C) The release of ROS and NO can induce tumor cell death, but conversely, through ROS, NO, arginase (ARG), prostaglandin E₂ (PGE₂), or a “shielding” effect of NETs, neutrophils can suppress cytotoxic immune cell activity.

Figure 2. Overlap in state-of-the-art TME imaging approaches.

Current state-of-the-art high-resolution imaging techniques allow highly multiplexed imaging in two dimensions with mass imaging or CODEX (Akoya Biosciences) and, to a lesser extent, spectral imaging. It is possible to image large volumes of tissues and even whole organs in three dimensions using tissue clearing techniques in combination with light sheet, confocal, or multiphoton microscopy, but multiplexing options are currently sparse. To capture cell dynamics in vivo, imaging windows can be implanted in mice to image cells in situ in real time. However, tissue penetration and multiplexing options are again currently limited. The use of transparent organisms such as zebrafish embryos and the combination of volumetric imaging/intravital microscopy with spectral imaging could be a way to circumvent some of these limitations.